In order to allow the mould to be released from a moulded concrete body when the concrete body has been fully or partially set, it is necessary to apply a release composition to the mould before the moulding process, i.e. before the concrete composition is poured into the mould. The action of a concrete release agent is partly based on the principle that the curing of the concrete surface is delayed or even prevented so that the concrete body does not adhere to the surface of the mould. The delay in curing or the prevention of curing must only apply to a very thin layer of the concrete body so that the strength of the concrete body is not affected or is only affected to a minor extent.
Such compositions must fulfil various demands, i.e. they must be able to adhere to a certain amount to the mould, they must impart a retarding influence to the surface layer of the concrete, they must have a suitable viscosity index so that they can be sprayed on the surface of the mould both in winter and in summer temperatures, and they should have a minimum effect on the environment.
Another way of obtaining a release ability is to apply a hydrophobic release composition so that the cured concrete will not adhere to the mould.
The release compositions used previously were normally based on mineral oils, and as additives there were normally used kerosene to act as a viscosity decreasing agent, retarding agents for improving the release properties, and other additives which may be wetting agents, adhesives and corrosion-protective agents. Normally, known release compositions contain 65-99% by weight of mineral oil and kerosene and 1-35% by weight of additives. A preferred oil component is spindle oil having a viscosity of about 20 mm.sup.2 /sec. (CSt) at 40.degree. C. The kerosene used will normally have a boiling point of 150-200.degree. C.
However, it is well-known that the use of mineral oils involves a health risk causing toxic and allergic exzema, skin irritation and skin cancer, and when used in sprayed form, the mineral oils may cause lung diseases. In addition to the health risks connected with the use of mineral oils per se, there is also an environmental disadvantage as mineral oils are normally only slightly bio-degradable. Therefore, the widespread use of mineral oils as concrete release agents involves a considerable risk of pollution.
It has been suggested to use vegetable oils to wholly or partially substitute mineral oils in concrete release agents. German Offenlegungsschrift No. 2,253,497 describes a mixture for use in demoulding concrete and plaster comprising a mineral oil and/or a hydrocarbon and at least one glyceride and additionally comprising a surfactant derived from a vegetable or animal fat. The use of surfactants permits the formation of a thin uniform film. The effect of glycerides is to form calcium salts or calcium-containing soaps which are only slightly soluble in water and prevent the curing of the concrete. However, glycerides are often too reactive (they have too strong a curing-preventing activity) to be used in mould release agents as it is difficult to modify their release properties. Hence, glycerides will often yield a porous surface layer caused by the prevention of curing in the outer layer. The use of glycerides is furthermore restricted by their high viscosity. Glycerides of higher saturated fatty acids are high-melting so that they will at normal temperatures separate from solutions based on mineral oils. So in spite of their harmlessness and their bio-degradability, their use is limited.
In order to impart low viscosity to release agents comprising mineral oils and/or vegetable oils, solvents were normally added. A suitable viscosity for applying mould release agents on moulds is in the range of .ltoreq.35 cP at 20.degree. C.
Japanese Patent Application No. 50-97840 (Nippon Seikiyu K.K. and Mitsuo) discloses mixtures of free fatty acids and esters thereof which are used as retarding agents in release oils on a mineral oil basis. The oily agent (the fatty acids and esters) and the mineral oil are used in a weight ratio of 1:1-20, the oily agent containing (a) 50-96% by weight of at least one component selected from C.sub.12-20 saturated and C.sub.18-22 unsaturated fatty acids and (b) 50-4% by weight of at least one component selected from fatty acid esters of C.sub.12-20 saturated and C.sub.18-22 unsaturated fatty acids with C.sub.1-18 monovalent alcohols. Hence, the retarding agent comprises at least 50% by weight of a mineral oil and at the most 25% by weight of a fatty acid ester.
In the Japanese application, it is described that combinations of certain fatty acids and certain esters in combination with a mineral oil, provide advantageous effects as mould release agents. Specifically, the methyl ester of bovine fatty acid in admixture with a mineral oil is described as a comparison. However, methyl esters of fatty acids are in fact characterized by their very strong retarding effect so that the esters, when added in only small amounts, increase the release effect of the mineral oil, but cannot substitute the mineral oil.